1. Field of the Invention
The present invention relates to an optical fiber amplifier. This specification is based on Japanese Patent Applications Nos. 2000-49942 and 2000-266256, the contents of which are incorporated herein by reference.
2. Background Art
Recently, an optical fiber amplifier which uses a rare earth doped optical fiber, comprising a rare earth element such as erbium (Er) and praseodymium (Pr), doped to the core of an optical fiber comprising quartz glass or the like, has been developed to a level where it can be practically used.
The optical fiber amplifier using an Er-doped fiber (the Er-doped optical fiber amplifiers) outputs with particularly high saturation and high gain in 1.55 xcexcm band. For this reason it is applied in various commercial systems, such as basic transmission systems and subscription systems.
Most noticeably, the optical fiber amplifier is being applied in WDM (wavelength division multiplex) transmission using more than several waves of signal light. There is a demand to improve the amplification characteristics of the optical fiber amplifier, with the aim of obtaining high-performance WDM transmission.
The Er-doped optical fiber amplifiers (EDFAs) are required to have high output power, wide bandwidth and flat gain profile. Especially, the gain flatness of EDFAs is a very important factor in wide-band long-haul WDM transmission systems. There are currently several different techniques for passive gain equalization, for example, a long period fiber grating, a discrete type fiber and a planer lightwave circuit type filter. These positive gain equalizers are designed to certain fixed operating conditions of the Er-doped optical fiber amplifier (EDFA), therefore the EDFA with the passive gain equalizer cannot adapt itself to changes in the operating conditions. The gain flatness deteriorates when the gain of EDFA changes due to variations in the number of signal wavelengths or in transmission fiber loss. To cope with the deterioration of gain flatness, several techniques have been proposed to maintain the gain flatness actively, such as the techniques described in xe2x80x9cOptical Amplifiers and their Applications 1995 Lecture Number FC3xe2x80x9d and xe2x80x9cOptical Amplifiers and their Applications 1999 Lecture Number WC5xe2x80x9d.
The former optical fiber amplifier comprises a variable attenuator inserted between two Er-doped optical fiber amplifiers. Here, xe2x80x9cEr-doped optical fiber amplifierxe2x80x9d represents an optical fiber amplifier which broadly comprises an Er-doped optical fiber, a pumping light source which inputs pumping light to the Er-doped optical fiber, a coupler, and the like.
In this optical fiber amplifier, the gains of the first and second Er-doped optical fiber amplifiers are made constant. Therefore, the gain/wavelength characteristics do not vary, even when the power of input light (signal input power) varies.
Furthermore, since the variable attenuator has no wavelength characteristics, the gain/wavelength characteristics do not vary from the first Er-doped optical fiber amplifier to the variable attenuator, or after passing the variable attenuator.
The output power of the optical fiber amplifier varies unless the variable attenuator is controlled. Therefore, the attenuation of the attenuator is changed by an amount equivalent to the variation in the signal input power in order to keep the output power constant. As a result, the optical fiber amplifier maintains gain flatness, even when the signal input power varies.
The latter optical fiber amplifier comprises a variable loss slope attenuator, instead of the variable attenuator mentioned above.
However, these optical fiber amplifiers require control systems and variable attenuator to keep gain flatness and output constant. Consequently, they have a drawback that their constitutions are complex and expensive. In addition, they are not sufficiently reliable.
It is an object of this invention to provide an optical fiber amplifier which can obtain gain flatness in a wide wavelength band. It is another object of this invention to provide an optical fiber amplifier in which gain flatness does not vary in a wide gain dynamic range, thereby maintaining gain flatness even when the input signal light varies. It is yet another object of this invention to provide an optical fiber amplifier which has a simple constitution and is inexpensive. It is yet another object of this invention to provide an optical fiber amplifier which has high reliability.
In order to achieve the above objects, the optical fiber amplifier according to this invention comprises an optical fiber amplification section; and an optical fiber for light loss which is provided in a later stage than the optical fiber amplification section or midway therein. The amount of light absorption and the wavelength dependency of the absorption (i.e. absorption characteristic) of the optical fiber change according to the power of input light.
This invention obtains the following effects.
It is possible to provide an optical fiber amplifier which can maintain gain flatness in a wide wavelength band, the gain flatness is not changing over a wide dynamic range and being maintained irrespective of variation in the power of input signal light.
Further, the optical fiber used in light absorption is comparatively inexpensive, has a simple constitution and control, and high reliability. Therefore, the optical fiber amplifier can be made inexpensive and highly reliable.